|  | 8.2 | Network Security
| 8.2.4 | Redundancy techniques

The next method of protecting data is through fault tolerant storage devices. This type of redundant set of devices is categorized by RAID (Redundant Array of Inexpensive Disks) levels 0-5. All of the basic RAID types will be shown but we will look specifically at the three levels of most importance. The types are as follows: RAID 0 Stripes data across multiple disks, no parity, so there is no redundancy. RAID 1 Disk mirroring (disk duplexing) writes data to two identical partitions on separate hard disks thus creating an automatic backup. Disk duplexing uses two hard disk controller cards as well as two hard disks to prevent the controller card as being the single point of failure for the system as in disk mirroring. RAID 2 Writes data across multiple hard disks, with error checking. This system is not used any more because it requires expensive disk modifications to work. RAID 3 Stripes data one byte at a time and has a dedicated parity drive. A good, but expensive redundancy choice. Because of the expense, this solution is not used very often either. RAID 4 Stripes data one sector at a time and has a dedicated parity drive. An expensive redundancy choice that is very slow on data writes to the disk. Because of the expense and the slowness of writing, this solution is not used very often either. RAID 5 Stripes data and parity across multiple disks (at least three for RAID 5). By mixing the parity across all of the disks, a separate parity disk is not required and yet full data redundancy is achieved. Data writes to the disk are still slow, but the cost isn't so high. One other important fact about RAID 5 is that on a Windows NT system the boot and system partitions cannot be located on a RAID 5 disk array. There are other RAID levels but they are beyond what is needed for the level of understanding we need. In fact not all network operating systems support the RAID levels mentions above. The three RAID levels that are supported by most operating systems are RAID 0, RAID 1 and RAID 5. The key points to remember are RAID 0 just is used for speed and provides NO data redundancy (backup). RAID 1 provides full data redundancy, but requires twice as much storage space, because all data must be written to two separate disks, and still has a single point of failure in the controller card. This problem is taken care of by the other variation of RAID 1, which is disk duplexing, where the disk controller is duplicated also. RAID 5 requires a minimum of three disks (in a WinNT system, four because the system and boot partitions cannot be on the RAID set) and the partition size must be the same on each disk. RAID 5 is popular because it provides very fast data reads from disk, which gives better throughput to the network. One last important point about RAID 5 and Windows NT, to have full redundancy, this must be done. You need at least five disks, the first two will be set up as RAID 1(disk mirroring) for the system and boot partitions, and the last three data disk will be set up with RAID 5. This will provide full redundancy, with the speed advantage that RAID 5 will provide. The last term that you will see when working with hard disk storage is volumes. A volume is a term that refers to a physical unit of storage. A good analogy would be to think of an encyclopedia set. What is each book normally called in the set, a volume. Some books hold more that one volume, for example, the XYZ volume. Finally, think of the whole set of encyclopedias as a single unit, this concept is true of the disk volume set also, one volume name that includes space from multiple disks that are referenced as a single unit. This information is important because volume sets are used quite often in network systems. One last thing about volume sets is that they provide NO data redundancy; they are just a way to refer to large storage areas as a single unit.
| Web Links
| RAID Info
| Raid Info Utilizing Graphics
| Where RAID Excels
| What is RAID?
| The Many Equations of RAID